Continuing genetic and cytological investigations of the filamentous ascomycete Neurospora will concern chromosome rearrangements - their origin, behavior, and applications; segregation-distorting Spore killer mutants - their chromosomal basis, mode of action, and significance in nature; and chromosome behavior in meiosis, as revealed by the synaptonemal complex and recombination nodules of nuclei reconstructed from thin sections in crosses that involve chromosome rearrangements and meiotic mutants. Mutants affecting events in the ascus will be analyzed. A locus-specific mutator system will be investigated, and other example of instability will be examined. - Genetic analysis of natural populations will be continued, using 4000 strains previously collected from five continents, with special interest in meiotic drive, the genetic isolation mechanisms involved in speciation, and dysgenic consequences of crosses between unrelated strains. -Aims of the research are to develop Neurospora as a model organism for genetics, cytology, and molecular biology; to increase understanding of chromosome organization in a small eukaryote genome; to analyze recombination, meiosis, and other events in the developing ascus; and to extend population genetics to organisms having a predominantly haploid life cycle. Knowledge of Neurospora Segentel aneuploids that are generated in meiosis parallel chromosomal anomalies responsible for birth defects in man.